First, please note the effect only applies when the men find the women attractive. If you were to find women they are not attracted to, seeing them kiss will have a very different effect.

But, given that both women are attractive, then the reason men like it:

1) The idea of a threesome with both of them.

2) It's women being sexual without a man around, so you aren't jealous /other negative feelings associated with other men being sexual.

3) The more advaned version of 2 is this: Some men have been taught that they are male, and therefore unattractive. In their heart they wish to be attractive, which in their mind, is female. They are projecting themselves as one of the attractive people kissing.